This invention relates to electronic ovens which include both high frequency energy means as well as conventional electric heating units, together with a self-cleaning capability, and more particularly to an improved shelf arrangement for such ovens.
Electronic ovens utilize high frequency electromagnetic wave energy to heat directly and internally foodstuffs placed therein, whereas conventional electric ovens utilize radiant energy and circulating heated air for heating the external surfaces of the food to be cooked. In self-cleaning ovens, the oven liner and the elements disposed in the heating cavity are heated to a high temperature up to 1,000.degree.F., well above the temperature range at which foods are normally cooked, for pyro-cleaning of the oven.
One of the problems presented by electronic ovens, and particularly those equipped with conventional electric heating units as well as a source of high frequency energy, concerns the shelves which are needed to support the food to be cooked. Because metal objects may absorb a substantial amount of the microwave energy supplied to the oven and also may distort the electric and magnetic field patterns so as to cause uneven cooking, shelves made of non-metallic dielectric materials have been used in some electronic ovens. However, these dielectric shelves are more costly than metal shelves, such as wire racks used in conventional ovens, and have proved to be quite unsatisfactory in combination electronic-electric ovens because of their poor heat resistance. This problem is particularly acute in self-cleaning ovens because of the extremely high temperatures reached during the self-cleaning operation. At present, there are no low cost dielectric materials which can operate over the temperature range experienced in a self-cleaning oven.
Accordingly, metal racks or shelves must be used in a self-cleaning oven. Metal shelf arrangements have been devised for use in electronic ovens. One such arrangement is disclosed in the U.S. Pat. No. 2,912,554 issued to F. R. Snyder on Nov. 10, 1959, and assigned to the assignee of the present invention. The Snyder patent discloses a metal oven rack having side rods which are spaced a predetermined distance from the side walls of the oven heating cavity, the rack being mounted on the side walls by means of support rods which are connected to the rack side rods by short connecting links respectively disposed at points of minimum induced voltage between the rack and the heating cavity side walls for the particular wavelength of microwave energy being used. But the principle of operation of the Snyder arrangement is not disclosed in that patent and it appears that the arrangement is effective only for the specific configuration illustrated in the patent, wherein the rack is disposed between the microwave antenna and the food to be heated. Furthermore, this arrangement does not permit the flexibility of locating the food at different positions in the oven heating cavity.